Automotive exhaust gases consist mainly of carbon monoxide (CO), hydrocarbons (HC) and various nitrogen oxides (NOx) as pollutants. In order to remove these undesirable compounds, catalytic converters have been employed which have more or less catalytic activity for the simultaneous oxidation of CO and HC and reduction of NOx. The conversion of the pollutants is performed preferably under stoichiometric conditions, which means that the oxidizing and reducing constituents of the exhaust gas are just balanced so that oxidation of CO and HC and reduction of NOx to harmless carbon dioxide, water and nitrogen can be performed simultaneously. For conventional fuels the oxygen content of the exhaust gas under stoichiometric condition is around 0.7 vol.-%.
The λ-value is defined as the air/fuel ratio (A/F) of the exhaust gas normalized to stoichiometric conditions. The air/fuel ratio for stoichiometric combustion of conventional gasoline and diesel fuels is approximately 14.7 which means that 14.7 kilograms of air are needed to burn 1 kilogram of fuel completely. The λ-value at this point is λ=1. Depending on the load and revolution, common gasoline engines usually operate with periodic fluctuations at λ-values around λ=1. This can be achieved by a so-called lambda-sensor control. For this application, so-called three-way catalysts are widely used for exhaust gas aftertreatment.
Three-way catalysts comprise a heat resistant carrier formed of cordierite or metal, a high surface area catalyst support, e.g. γ-alumina, and at least one precious metal element of the platinum group elements which is supported on the catalyst support. In order to enhance the conversion level of oxidizable compounds, an oxygen storage material on the basis of cerium oxide is used.
Oxygen storage materials are able to store oxygen in oxidizing atmosphere or release oxygen under reducing conditions, respectively. The storage and release of oxygen is associated with a change of the oxidation state of Ce3+ to Ce4+ and vice versa. The amount of oxygen uptake or release as well as the adsorbing/desorbing kinetics under dynamic exhaust conditions are strongly dependent on the chemical composition, synthesis conditions and structural parameters of a given material.
In the future, more stringent exhaust emission regulations will lead to an increased demand for oxygen storing materials with improved oxygen storage capacity as well as higher thermal stability. Particularly, so-called close-coupled catalytic converters, which are positioned close to the engine, may reach temperatures up to 1100° C. when the engine runs under full load. Under these severe conditions the primary particles of the oxygen storing materials usually tend to sinter to form larger agglomerates that lead to a loss of surface area as well as oxygen storage capacity, and thus result in a decrease of catalyst purifying activity.
It is known in the art that impregnating bulk ceria or a bulk ceria precursor with a liquid dispersion of an aluminum-stabilizer precursor, and calcining the impregnated ceria, gives improved thermal stability.
Furthermore, it is known that oxygen storage materials show higher resistance against sintering and a significant higher oxygen storage capacity when they are highly dispersed on the specific surface area of a thermally stable support oxide with a high surface area such as alumina.
The prior art discloses a composite oxide support and a process for its preparation based on alumina with at least one member of the group consisting of ceria, zirconia or ceria-zirconia. Additionally, the described composite oxide may contain barium or lanthanum.
To manufacture the composite oxide support according to the prior art, a solution of salts of a plurality of elements including at least one of cerium and zirconium, and aluminum, which define the composite oxide, is first mixed with an alkaline solution with the use of high speed mixing means to form a precursor of oxide composed of the plurality of elements. The precipitate is first dried and then calcined in air at 650° C. for 1 hour. To achieve a high mixing speed, a high rotating agitator is used. One substantial disadvantage of the described process is the use of alkaline hydroxides, which cannot be completely removed from the product.
The prior art also discloses a composite oxide and a process for its preparation consisting of an oxide of a metal M1 of the group of Ce, Zr, alkali earth or rare earth metals in an amount of at least 50% per weight based on the total weight of the composite oxide, and an oxide of a metal M2 of the group of Al, Ti or Si, whereas the metal oxide M2 is not soluble in the oxide of metal M1 and both metals are dispersed at the nanometer level. The oxides of the metals M1 or M2 additionally may contain a further oxide of a metal M3 of the group of Zr, alkaline earth or rare earth metal. The material is prepared by mixing suitable precursors of the metal oxides in the desired amount and precipitated by addition of an aqueous ammonia solution, dried and finally calcined.
Based on the current state of the art, there is still a need for an oxygen storage material containing ceria with a high specific surface area after thermal aging and an improved oxygen storage and release capacity under dynamic exhaust conditions.